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While fairly common in migration, the foliage-loving Philadelphia vireo can easily go undetected. Keith Corliss

Philadelphia vireo flies under the radar

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One of the appeals of destinations such as Yellowstone National Park is the better-than-even chance of seeing any of a number of spectacular species of wildlife amidst the backdrop of grandiose scenery. On any visit there a family might witness elk, moose, bison, gray wolves, or bighorn sheep. For things like grizzly bear or mountain lion the odds fall off rather quickly. Those animals - for various reasons - are quite a little more secretive.

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A similar scenario plays out with watching birds as well. There are the in-your-face species a person absolutely cannot miss. American robins come to mind. So do house sparrows. European starlings must be included in that group too. At the other end of the spectrum are the "mountain lions" of the bird world; the ones which, while not necessarily rare, seem to fall under the radar and go unnoticed.

A certain little songbird is one such critter. It's a member of a genus whose job it seems, is to avoid detection by any means other than auditory. And this little beauty seldom sings around here; it's merely passing through. But careful and deliberate observation in woodsy areas will eventually produce a Philadelphia vireo (Vireo philadelphicus) or two during migration.

As a whole, vireos are perhaps the least recognized group of common songbirds around. They largely go unseen. The birds are berries-and-bugs specialists, keeping to dense brush and leafy trees and typically not appearing in spring until the trees have leafed out. Given their reliance on caterpillars, this only makes sense.

Of the 14 or so species found in the U. S., six are regulars to North Dakota, mostly in late May. It's not out of the question to see four or five of them in one spring day. (Bell's vireo is restricted in this state to specific sites along the Missouri River south of Bismarck).

Vireos tend to be somewhat drab but husky birds with short thick blue-gray legs and a stout hooked bill. One, the red-eyed vireo, is considered the most common songbird in the eastern U.S. This is curiously ironic given the fact that most non-birders have probably never seen one.

Philadelphia vireos nest from central Alberta in an easterly swath to the Canadian Maritimes where it favors early-to-mid successional deciduous forests, i.e. not large trees. In North Dakota the bird is a regular breeder in the Turtle Mountains area along our northern boundary. Speaking of north, of all the vireos the Philadelphia is the most northerly breeder.

The bird is olive-gray on the back with a slightly darker cap. It has no wingbars (which is one trait to separate vireos). The bird's breast is somewhat white with pale yellow on its flanks, under its tail, and on the throat. A dark eye, with a dark line running through it, is set on a pale gray face with a white line above the eye.

Here's where things get a little hard even for birders: Its song is very much like that of the more common red-eyed vireo. Cornell University's Birds of North America Online goes so far as to say, "As a result of this similarity, the Philadelphia vireo's presence in a forest often goes undetected even by experienced observers." The red-eyed vireo sings a series of notes in the manner of the American robin but broken into distinct phrases. The Philadelphia's song is like this but is slightly higher pitched and delivered with a slower tempo. Fear not, with practice the difference is noticeable.

So what we have in the Philadelphia vireo is an unnoticed and understudied bird in a family of unnoticed and understudied birds. Consider it a challenge then, to spot this fairly common migrant. Sort of like finding a grizzly bear in Yellowstone.

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